1. FIELD OF THE INVENTION
The present invention relates to a control system for hydroelectric power stations. More particularly, invention relates to a control system for hydroelectric power station systems of the type in which reservoir means is disposed between other two reservoir means provided upper and lower sites, respectively, these three reservoir means are connected in cascade by means of water channel, the intermediate reservoir means having smaller water storage capacity than those of upper and lower ones, and hydroelectric power stations are installed on the water channel connecting the intermediate reservoir means and that connecting the intermediate and lower reservoir means, respectively.
2. DESCRIPTION OF THE PRIOR ART
The demand for electric power greatly varies with hours in a day and with seasons in a year. For example, the demand for power may have a peak for a shorter period in day-time and be reduced greatly in mid-night. Therefore, it is desired that the operation of power stations is controlled to meet with such variation of the demand. Thermal or nuclear power stations are not suitable to be operated for power supply according to the power demand variations, since these stations are difficult in adjusting the generator and their efficiencies are low unless they are operated substantially at full load. It is for that reason that the thermal or nuclear power stations are used for supplying the steady base part of the demand for power and the hydroelectric power stations, which are relatively easy in adjustment of their outputs, are used for supplying the variable part of the demand. At present day most of advantageous and satisfactory sites for hydroelectric power stations have been already developed. For this, it is very difficult to find new sites where it would be possible to construct new hydroelectric power stations having desired capacities. Therefore, attention is now being paid to the so-called pumped storage power station, in which the water once used for driving the generator is stored in a lower reservoir and pumped up to and stored in an upper reservoir, the stored water being reused whenever it is needed.
The pumped storage power station uses water wheels and pumps or pump-water wheels disposed in the water channel connecting the upper reservoir with the lower reservoir. At the time of great demand for power, the water wheel is driven for power supply while, at the off-peak time in midnight, the pump is driven by using surplus powers in the power system for pumping up the water stored in the lower reservoir to the upper reservoir. The construction of such a pumped storage power station, however, is restricted on the ground of geological of topographical problems. One of the geological problems is that the lay of the site does not allow to construct the upper and lower reservoirs having a head or lift therebetween which is not too large, as compared with the capacity of the pumps and waterwheels of hydroelectric power station. A typical topographical problem is the difficulties of the works for installing water pressure tubes for guiding the water to the power station or of the engineering works for the power station construction.
For reducing these restrictions, there has been proposed a hydroelectric power station system in which an additional reservoir is provided between the upper and lower reservoirs, and power stations are installed between the upper and intermediate reservoirs and between the intermediate and lower reservoirs, respectively.
So long as the construction cost is concerned, it is undesirable to additionally construct the intermediate reservoir in the pumped storage power station system. For this, it is required to have the water storage capacity of the intermediate reservoir as small as possible. However, if the water storage capacity of the intermediate reservoir is very small in order to reduce its construction cost, the water level of the intermediate reservoir will greatly change, depending on the operation conditions of the respective power stations, that the stored water increases too much resulting in overflows or decreases to an extremely low level. This is unavoidable since it results from the difference of the "water-flow quantity-head" characteristics or the efficiency between the pum-wheels installed in the respective power stations. This problem may be readily solved, in the case of mere pumping up of the water. By monitoring and controlling the water level of the reservoir during the pumping operation so as to prevent such overflow or shortage of the stored water. In the case of operating the system for power generation, however, if the respective power stations were operated so as merely to keep constant the water level of the intermediate reservoir, the loads imposed on the respective power stations should necessarily change and thus it would be impossible to provide a stable power supply matching to the demand for power. Conversely, if the control were made to operate the respective power stations mainly for power generation matching to the power demand, the water level of the intermediate reservoir should greatly change.